Welcome to the Campfire.
I take it your cases are 3-1/4 inch?
Your rifle is probably twisted to shoot a max 350-360 grain bullet.
Buy or cast and size them to .458 inch, have or lube them with a good black powder lube.
Loading 120 grains of any 1F black powder will keep you in a safe spec spot provided the rifle has been Gunsmith inspected and deemed safe to shoot, you can then adjust powder slightly up or down to find sight regulation provided this is a double rifle.
A set of dies
Brass
A drop tube
458 cal powder compression die
458 cal taper crimp die
30 thousandths 458 cal Walters fiber wads
458 cal brass cleaning jag
Hickory wiping stick
458 cal cotton cleaning patches
Most all of these components can be purchased from Buffalo Arms.
I bet Montana Bullet Works has a bullet for you, they'll size em to 458 and lube em with black powder lube.
Good luck and happy shooting/hunting.
